1. Field of the Invention
The present invention is directed to a system for the registration of radiation images of the type, having a radiation pick-up device and a control device that controls the operation thereof.
2. Description of the Prior Art
Systems of the above type for the registration of radiation images are utilized, for example in medical technology, for the registration of X-ray images. They can be employed in ordinary X-ray systems; however, employment in computed tomography is also possible.
The central element of such a system is the radiation pick-up device. Components referred to as a-Si-panels that have a scintillator layer, mainly of Csl, are for use in such a known device. The incident x-ray quanta are converted into visible light therein, and the light is processed by a following, amorphous semiconductor layer wherein a matrix of photodiodes is fashioned. This matrix is followed by dedicated read-out electronics. Dependent on the quantity of light arising from the conversion, charge, i.e. electrons, is generated in the photodiodes, which is read out via the dedicated read-out electronics. A problem associated with such radiation pick-up devices is the presence of non-variable light coupling between the scintillator and the photodiode matrix. This causes there to be hardly any possibilities for variation of the conversion efficiency (incident x-ray quanta to output voltage of the panel). This means that no variation of the conversion efficiency or of the gain can be achieved given different operating modes or different pick-up modes that operate with x-radiation having different doses. In medical technology, for example, modes for producing fluoroscopic or transillumination exposures, and digital radiography or digital subtraction angiography exposures, are often implemented in alternation.
The first-cited operating mode operates with a low x-ray dose with simultaneous pick-up of many images; the latter operating modes operate with x-rays of a high dose per individual image that is registered. Since no variation of the conversion efficiency is possible given known a-Si panels, these panels must be selected such that no over-modulation occurs given pick-up of images having a high radiation dose. This, however, causes an increase in electronic noise for fluoroscopic or transillumination exposures, particularly compared to known x-ray image intensifier video systems.
Alternatively to such a-Si panels, radiation pick-up devices are known that employ a layer referred to as a HARP layer (HARPxe2x80x94high gain avalanche rushing amorphous photoconductor). Such a HARP layer is composed of a charge layer that generates electrical charges dependent on the incident x-rays and an electrode layer allocated thereto that is chargeable with high-voltage for triggering an electron-multiplying avalanche effect in the charge layer via, causing a potential to arise in the high-voltage-condition. The read-out ensues using an electron beam that scans the HARP layer. Such a radiation pick-up device is known, for example, from German OS 44 10 269. In this radiation pick-up device, a high-voltage is connected between the electrode layer and the emitter cathode that generates the electron beam. This high-voltage causes high electrical fields to arise in the charge layer, which is preferably composed of amorphous silicon. The electrical fields ultimately produce an avalanche effect in the amorphous semiconductor charge layer. This multiplies the electrical charges exponentially for increasing the electrical potentials. Strong electrical fields are required in order to generate this avalanche effect, but such fields are able to be achieved in a relatively simple way by making the charge layer extremely thin. A considerable signal gain can in fact be achieved as a result; however, this known system is likewise a rigid system that does not allow any variation in gain.
An object of the present invention is to provide a system of the type initially described that allows the gain to be adapted to the image exposure mode which is to be implemented in a simple way.
This object is inventively achieved in a system is provided for the registration of radiation images, having a radiation pick-up device and a control device controlling the operation thereof, wherein the radiation pick-up device has: a charge layer that generates electrical charges dependent on the incident radiation and an allocated electrode layer chargeable with high-voltage for triggering an electron-multiplying avalanche effect in the charge layer by producing a potential across the charge layer, a read-out device for reading out the generated charges in the charge layer by means of an electron beam, and wherein the potential across the charge layer can be varied for varying the gain of the charge layer caused by the avalanche effect.
The invention is based on radiation pick-up device as disclosed, for example, by German OS 44 10 269. For solving the aforementioned problem, the invention proceeds from the perception that a variable gain for the signals that are generated on the basis of the incident x-radiation can be achieved by varying the potential across the charge layer. By varying the voltage via the charge or HARP layer, the local gain due to the avalanche effect can be varied. The avalanche effect, i.e., its intensity, is dependent on how large the potential is between the free surface of the charge layer and the coupled electrode layer. The avalanche effect is more pronounced the higher the potential is and vice versa.
This allows that the inventive system to adapt the gain of the radiation pick-up device to the currently selected image exposure mode in a simple way. When, for example, it is necessary to register transillumination images with a low x-ray dose and radiography images with a high radiation dose, then the gain can be switched between the two different operating modes by a corresponding variation of the layer potential. Given image registrations with low radiation dose, a high gain is selected; a lower gain suffices given exposures with a low radiation dose.
The layer potential can be varied in a simple way by varying the high-voltage that is applied to the electrode layer, controlled by the aforementioned control device. The voltage can be varied either before or during the registration of a radiation image, by setting the free surface of the charge layer to a pre-selected potential. Additionally, the phenomenon that the potential across the charge layer is somewhat reduced dependent on the of induced charge carriers can be used to advantage, so that a gain reduction arises by itself during the exposure, even though it is slight. Since the curve of this gain reduction is known by virtue incident quantity of charge carriers, a linear amplitude characteristic can be determined, producing the advantage that there is hardly any over-variations; further, any variation range can be optimally scanned in view of the signal-to-noise ratio.
The amplitude of the high-voltage that is applied preferably should be continuously variable dependent on the dose of the incident radiation in order to thus be able to optimally adapt the gain to the operating mode employed.
The electrode layer can be arranged on a film-like carrier, particularly on a glass film, such as by printing, and can be composed of a of essentially parallel layer strips spaced from one another. A closed electrode layer surface, however, is also conceivable.
The read-out device can be a flat emitter device, so that an extremely low overall structure of the radiation pick-up device is achieved. The surface emitter device can have linear electron emitter cathodes having allocated horizontal and/or vertical deflection electrodes. Alternatively, the surface emitter device can have micro-structured electron emitter cathodes arranged in a matrix or an array, for example in the form of nano tubes or small emitter micro-tips.
It is expedient for the radiation pick-up device to be integrated in a flat vacuum housing wherein stabilization elements, particularly in the form of structural webs, are provided, to intercept the significant high forces that act between the large-area sides.
It is also expedient to provide at least one reset light source for the exposure of the charge layer, which should be capable of being operated in pulsed fashion via the control device. Using this reset light source and given simultaneous activation of one or more or of all electron emitters, it is possible to stabilize the free surface of the charge layer to a lower potential compared to the potential that was previously present. As a result, it is possible in a simple way to lower the sensitivity of the radiation pick-up device before the registration of radiation images having a high dose that were preceded by registrations having a low dose, wherein, thus, registration was carried out with a high gain.
Overall, the inventive system offers several advantages. First, the employment of a-Si panels provided with photodiodes having allocated switches can be foregone, since the inventive system and radiation pick-up device operate with an electron beam that scans line-by-line. This has the advantage that parasitic capacitances are minimized due to the elimination of the switch capacitances with a simultaneous increase of the fill factor and of the maximum charge that can be scanned (since the scanning electron beam allows an enhanced voltage boost of the pixels). The avoidance of the photodiodes, further, is advantageous because a significantly more beneficial inertia behavior is established, particularly when switching between the various operating modes. The afterglow behavior of known a-Si panels is essentially defined and dominated by the inertia behavior of the a-Si photodiodes, which are no longer present in the inventive system.
The employment, in particular, of micro-structured flat emitter cathodes also leads to lower acquisition costs and devices having a longer service life. The greater range of dynamics established due to the possibility of varying the gain also allows employment in multi-line computed tomography detectors as well as in x-ray photon-counting detectors.